1. Field of the Invention
The present invention relates to a method as well as a circuit configuration for mixing a digital signal with an analogue signal.
2. Related Background Art
For mixing a digital signal with an analogue signal it is known to convert the digital signal into an analogue voltage signal, which is received by the mixer after being filtered. Since mixers are usually provided with current inputs, prior conversion of the filtered output signal from the digital to analogue converter into a current signal is necessary. A known device of this kind is represented in FIG. 3. In this case, a digital signal 4 is received by a digital to analogue converter 1. The digital to analogue converter 1 supplies a current signal 5, which is received by a resistance 11, so that a voltage signal, which is filtered in a filter 2, preferably a low-pass filter, lies on the resistance 11. The filter 2 supplies a voltage signal as output signal 6, which is converted in a voltage to current transformer 9 into a corresponding current signal 10, which is received by the mixer 3. Prior conversion into the current signal 10 is necessary, since the mixer 3 has a current input. The mixer 3 further receives a second signal 7, with which the digital signal 4 is to be finally mixed, and at its output supplies a mixing signal 8.
A disadvantage with the circuit configuration described is that the current signal 5 at the output of the analogue to digital converter 1 leads at the resistance 11 to voltage increase, which reacts on the output of the analogue to digital converter 1 and in this way modulates the current signal 5 at the output of the digital to analogue converter 1, whereby its output signal is disturbed. A further source of disturbance is caused by the necessary voltage to current transformer 9, which introduces further disturbances and in particular non-linearities. Furthermore, the components first necessary at the output of the digital to analogue converter 1 for current to voltage conversion and finally before the mixer 3 for voltage to current conversion means additional complexity, which has a negative effect on development, cost and production.
It is an object of the present invention to create a circuit configuration or a method for mixing a first, digital signal with a second signal, whereby low cost and little disturbance and signal distortion can be ensured.
The object according to the invention is achieved by a method with the features of claim 1 or a circuit configuration with the features of claim 7. The sub-claims in each case define preferred and advantageous embodiments of the present invention.
The entire circuit according to the invention is operated in the current domain or within the current area, for which purpose a digital to analogue converter with current output is used, which supplies a current input of a mixer without any current to voltage conversion.
Due to the few components, little noise, few distortions and low circuitry cost are achieved. The reason for this is that mixers with current input are normally used anyway and the digital to analogue converter deployed with systems of this kind mostly possesses a current output. A digital to analogue converter of this kind can have switched internal power sources, for example.
Preferably, the current output of the digital to analogue converter is terminated with a diode or generally with a non ohmic element, so that varying the current signal at the output of the digital to analogue converter results in no or only minor voltage fluctuation at the output of the digital to analogue converter. A component of this kind for terminating the current output of the digital to analogue converter, as in particular a diode, can preferably be arranged in the mixer, whereby advantageously a component existing in the mixer anyway is used for this.
In an advantageous refinement, a filter is inserted between the digital to analogue converter and the mixer, which however operates in the current domain or within the current area. The filter can be both passive and active. A passive filter in particular can be an LC filter.
If an active filter is used, trans-conductance amplifiers can be used, for example, in place of the operation amplifiers as active elements, which are normally used in the case of filters in the voltage area.
The filter used preferably has a low-pass characteristic, in order to eliminate higher-frequency fractions caused by digital to analogue conversion, but can also have any transmission characteristic. It is also conceivable to design the filter as a band-pass filter.
The solution according to the invention is advantageously used for wireless data communication, whereby the digital signal preferably represents an information signal to be transmitted and the second signal the carrier signal. Advantageously, the solution according to the invention finds application in mobile telecommunications, whereby the digital signal has digitally coded speech signals.